Chickens with 5 different phenotypes for the expression of endogenous avian C-type virus (K16 chf ion gs ion, K18 chf ion gs-, line 7 chf-gs- inducible for subgroup E virus, line 15 chf-gs- inducible for an uncharacterized avian C-type virus, and Kimber chf-gs-) will be used in breeding experiments to determine whether these phenotypes are inherited from linked or unlinked genes. The expression of the different phenotypes will be examined in heterozygotes to determine the dominance and look for possible interactions of the various genotypes. The horizontal spread of endogenous avian C-type viruses will be examined with particular emphasis being placed on characterizing the growth of the subgroup E virus and line 15 virus in chicken cells. The intracellular mechanism which appears to restrict the growth of endogenous subgroup E viruses in some subgroup E virus susceptible chicken cells (K28) but not in others (line 15) will be examined for its inheritance and mode of action. The line 15 virus will be characterized.